The fundamental objective of this project is to determine the differentiation program of murine natural killer (NK) and killer (K) cells and to define the modulating effects of neoplasia on the production of NK and K cells. In vivo cytokinetic studies with [unreadable]3[unreadable]H-TdR, repopulation assays, and in vitro colony-forming assays are being used to determine the proliferative status and renewal rate of NK and K cells in the bone marrow and their life span in the periphery. Since NK cells acquire binding ability prior to lytic ability, we determined the proliferative status of target binding cells (TBC) in marrow. Six-week-old C578L/6 female mice were injected intravenously with [unreadable]3[unreadable]H-TdR (1 micro Curie/gm body weight). Femoral marrow was obtained after 2, 12, 24, or 48 hrs and treated with anti-Ig and C to eliminate B cells. The remaining "lymphoid" population comprised 17% of the initial BMC. Cytospots were radioautographed and subsequently examined with phase contrast fluorescence microscopy to assess the percent TBC. Two size classes of TBC could be distinguished on the basis of cell diameter: large, greater than 9 micrometers, and small, less than 9 micrometers. The percent of H-TdR-labeled small TBC in the marrow was 8, 16, 27, and 16% at 2, 12, 24, and 48 hrs, respectively, whereas the percent labeled large TBC was 24, 18, 21, and 14% at the four time points. The increase in labeling index of small TBC during the first 24 hrs and subsequent decrease indicate that small TBC are newly produced cells with a rapid turnover. The large marrow TBC appear to be a self-maintaining precursor pool that gives rise to small TBC. We are continuing to test the effect of in vivo depletion of NK and K cells during the neonatal period using our model for specific depletion by in vivo treatment with NK 1.1 antiserum. In the future, we will test the effects of specific depletion-of NK cells on: (1) the production and maturation of NK and K cells, and (2) the induction and growth of 3-methylcholanthrene-\and Moloney sarcoma virus-induced sarcomas. (SR)